The present invention relates to attenuators, and in particular, to circuits and methods that may be used in wideband applications.
FIG. 1 illustrates a prior art attenuator. Attenuator 100 is known as an R2R ladder. In an R2R ladder attenuator, a plurality of resistor dividers are configured in series and the output nodes of each divider (i.e., the attenuator “taps”) may be coupled to a subsequent stage through switches 141-143. Each tap provides a different attenuation value. In an R2R ladder, resistors 110, 113 and 115-116 have the same value, and resistors 112 and 114 have the same value. Moreover, the value of resistors 112 and 114 is twice the value of the other resistors. Using this configuration, the resistance at each output node to ground is the same. This provides for successive attenuations steps of 6 dB per tap.
One problem with existing attenuators such as attenuator 100 is that the resistance values combine with input capacitance of subsequent stages and will cause the circuit to have a limited bandwidth. For example, if the output taps of attenuator 100 are coupled to the input of an amplifier 150 through switches 141-143, the load capacitance from the switches and from the input of the amplifier will limit the band width of the system. Thus, attenuator 100 may not be useful in wideband applications.
Thus, there is a need for an improved attenuator, and in particular, for wideband attenuator circuits and methods.